It is well known that steel and other metal surfaces can corrode in the presence of aqueous environments, especially acidic aqueous environments such as those found in subterranean wells. In this regard, aqueous fluids passing through such subterranean formations, and especially those found in hydrocarbon production wells, often contain high concentrations of corrosive materials such as dissolved gases, including hydrogen sulphide and carbon dioxide.
Alloy technology and galvanisation have resulted in materials that can withstand some incidental contact with corrosive environments, but in a number of industrial applications more prolonged contact with corrosive environments occurs. This is especially the case in industrial hydrocarbon systems, such as hydrocarbon exploration, recovery and refining. In particular, during the working life of an oil or gas well there are various conduits and other components in the production zone that encounter considerable corrosion.
Therefore during the life of an oil or gas production well, the well (and especially the production zone within the well) will be expected to experience problems with corrosion due to dissolved gases. For example, corrosion of metallic components, such as downhole tubulars, is common and is evidenced by surface pitting, localized corrosion and loss of metal. Metallic surfaces subject to such corrosion include carbon steels, ferritic alloy steels, and high alloy steels such as chrome steels, duplex steels, stainless steels, martensitic alloy steels, austenitic stainless steels, precipitation-hardened stainless steels and high nickel content steels.
Corrosion inhibitors are therefore widely used in oil and gas production wells and pipelines to reduce corrosion of metal components and to therefore prevent consequential production equipment failures.
Some of the most widely used surfactant corrosion inhibitors in oilfields involve nitrogen-based chemistry, for example they include those based on quaternary ammonium salts and fatty amine, fatty amidoamine, hydroxyethyl or aminoethyl imidazolines. For example, imidazolines are commonly used as corrosion inhibitors, and are viewed as the industry standard. However, these chemicals are known to have poor aquatic toxicity.
The OSPAR Convention (Oslo-Paris Convention for the Protection of the North-East Atlantic) has a control system in place, whereby a standard environmental dataset for each chemical substance in a formulation to be used in the North-East Atlantic Ocean is required. This is called the Harmonized Mandatory Control System (HMCS). The environmental dataset includes results for biodegradation, bioaccumulation and marine aquatic toxicity. Other conventions and regulations also determine environmental standards to be met.
These environmental criteria, and in particular, toxicity to marine algae, are typically not met by many substances used in commercially available corrosion inhibitors. For example, measurement of the effective concentration of the substance that is sufficient to cause a reduction in growth rate for more than 50% of the algae population (the EC50) is often less than 1 mg/l for in commercially available corrosion inhibitors, and in some cases less than 0.1 mg/l. In contrast, EC50 values should in fact ideally be >1 mg/l, more preferably >10 mg/l. In other examples, the commercially available corrosion inhibitors show poor biodegradation, or are considered likely to bioaccumulate.
Therefore although their performance attributes are well known, a major disadvantage for the above mentioned nitrogen-based surfactants is their environmental profile. In this regard, these compounds may have poor biodegradability, may have the potential to bioaccumulate in the ecosystem and are harmful or toxic to aquatic species.
Furthermore, cationic nitrogen-based surfactants may also have problems associated with being incompatible with other components, such as anionic corrosion inhibitors.
In addition, when modifications are made to such nitrogen-based surfactants to try to improve their environmental profile, e.g. when the hydrophobic chain length is reduced, this often has a detrimental effect on their corrosion inhibition performance.
There is therefore a continuing need for products for use in the oil and gas industry, which have acceptable environmental properties, especially in terms of aquatic toxicity and biodegradability, whilst also being effective in terms of corrosion inhibition, and preferably with a reduced foaming and emulsification potential.